Last time, I talked about how worthless a story idea is if you don't do anything with it. What if you have the opposite problem? Instead of thinking that your idea is great, what if you don't like it because you think it's a rip-off of someone else's idea? Well, to tell you the truth, it probably is. That's because, at some level, every story idea has already been taken.
I just read Battle Royale by Koushun Takami. It's about a government that forces a group of teens to kill each other until there's only one survivor left. Does that sound familiar? Hunger Games, anyone? Yup, when The Hunger Games grew in popularity, some critics accused it of ripping off Battle Royale, which was published earlier. On the surface, they are two very similar story ideas, but I believe Suzanne Collins when she said that she wasn't aware of the Japanese book and came up with her idea independently. Anyone who's read both books will see that both authors treated the subject matter in different ways, their writing styles are vastly different, and as a result, a reader can enjoy both books without feeling cheated by two seemingly identical story ideas.
To take it to an extreme, some say that there are only seven basic story plots. (I've also heard as few as three.) So what are the chances that the new story idea in your head is different from what every author who's come before you has written? Zero. But don't worry. You're not Hemingway, you're not Stephen King, you're not Suzanne Collins. And I mean that in a good way because it means that your take on the idea will be different from theirs. Despite the apparent similarities, if you're true to yourself and write the best story you can, it'll be sufficiently different from what other authors have written.
Now, I don't condone plagiarism or conscious rip-offs of other works, but there's a lot of room to work with before you reach that stage. Consider the number of sparkly vampire stories that popped up when Twilight became popular. Those authors were definitely trying to capitalize on Twilight's success by emulating it. I haven't read any of those books (nor Twilight for that matter, although I've seen the movies), but if they can be deemed different enough from the original, chances are that your story idea will be considered original as well.
Don't let the fear that your idea isn't unique stop you from pursuing it. As I mentioned before, an idea is worthless if it stays in your head.